The present invention relates to coherent energy beam imaging and, more particularly, to a novel method for adaptive formation of the coherent beam, using phase conjugation, to counteract the effects of inhomogeneous wave propagation.
It is well known that phase cancellation effects, introduced by inhomogeneous wave propagation in a medium, limit both the spatial and contrast resolution of images derived from a beam of coherent energy, such as a beam of radar, sonar or ultrasound energy and the like. Phase cancellation effects are most acute for high frequency imaging using large apertures, where small arrival time differences over substantial distances result in large errors in the relative phase across that aperture. For example, in medical ultrasound this occurs because the phase of an interrogating ultrasound wave is distorted due to inhomogeneous distribution of sound velocities in the body through which the ultrasound wave propagates, from the aperture to the region of interest and the subsequent return to the aperture. It is well known that this effect can dominate the appearance of an ultrasound image as both the ultrasonic frequency and the size of the aperture are increased. While several methods have previously been proposed for reducing phase cancellation effects, all such methods operate, if at all, at the expense of spatial resolution. In order to image at the theoretical resolution of the aperture, especially at high frequencies, phase distortion of the incident ultrasound wave must be reduced. In ultrasound imaging, the predominant distortion occurs within the wall of the body being imaged, which can be modeled as a single distorting surface. For all regions of interest deeper than this body wall, it is highly desirable to reduce phase aberration by obtaining accurate information from which phase correction can be computed.